Constraining λBₛ by Bₛ→ γ^* and Bₛ→ φ form factors

We calculate the form factors FV(q²,q'²) and FTV(q²,q'²) describing the Bₛ→ γ^* transition induced by the vector and tensor weak currents in a broad range of values of q² and q'² far below the quark thresholds in both q² and q'² channels. These form factors are calculated via the distribution amplitudes of the Bₛ-meson. We then interpolate the obtained results by a formula that contains pole at q'²=M_φ² and extract the residue which gives the Bₛ→ φ transition form factors V(q²) and T₁(q²). In this way we obtain theoretical predictions for these form factors without invoking quark-hadron duality and QCD sum rules. Furthermore, we calculate the relationship between $V(0)$ and T₁(0) and the parameter λBₛ(μ), the inverse moment of the Bₛ-meson distribution amplitude. Using the available predictions for $V(0)$ and T₁(0) coming from approaches not referring to the Bₛ-meson distribution amplitudes, we obtain the estimate λBₛ(μ mb)=(0.62± 0.10) GeV.